Particulate emissions of gasoline engines will be newly regulated in Europe in 2014 with the introduction of EU6a regulations of 6×1012 particles/km and further reduced to 6×1011 particles/km with the introduction of EU6c in 2017. Similarly, United States regulations will impose similarly challenging standards with the introduction of LEVIII. Standards are assumed to be 10 mg/mi in 2014, 3 mg/mi in 2018 and 1 mg/mi in 2025. A major source of particulate emissions is known to be from a diffusion flame fed by fuel evaporating from the deposits on the fuel injector tip.
It is known that protruding the fuel injector further into the combustion chamber reduces the particulate emissions. Increasing injector tip protrusion raises injector tip temperature by exposing more injector tip surface area to hot combustion gases. This in turn enhances evaporation of any fuel remaining on the tip so there is no or little fuel remaining on the tip to be ignited when the flame front passes. The higher tip temperature also enhances oxidation of the deposits on the tip reducing the sponge-like surface of the deposits which hold the fuel.
Increasing tip temperature enhances evaporation on the external surfaces of the tip lowering particulate emissions, but it also increases the temperature of the fuel metering orifices or passages. This increases the risk of deposits being formed in the metering passages themselves. It is well known that fuel characteristics, tip (orifice) temperatures, fuel pressure and nozzle design affect deposit formation in injector flow passages. It is generally accepted that if the tip temperatures are kept below 120° C., that no problems with deposit related flow shift will be encountered. This guideline is only achievable with side mounted direct injectors. In centrally mounted injector applications, temperatures up to 300° C. can be seen.
Thus, there is a need to increase the injector tip temperature to lower particulate emissions while allowing the metering passages of the injector to be cooled by the fuel to prevent deposit formation in the passagers and thus prevent flow shift.